Age & Sex of Chicks

Ok, we picked up these little Wheaten/Blue Wheaten Ameraucana's yesterday. I understood that we were getting day old chicks, hatched Wednesday. However, there is a huge size difference between the chicks. Two of the chicks are much larger, have a brown tint to their backs, feathers are much further along, and already have tail feathers. They even strut around differently. Could the two larger ones be roos or are they just older? We paid for all females.